Numbers 10:9
And if ye go to war in your land against the enemy that oppresseth you, then ye shall blow an alarm with the trumpets; and ye shall be remembered before the LORD your God, and ye shall be saved from your enemies. And if ye go to war in your land against the enemy that oppresseth you, then ye shall blow an alarm with the trumpets; and ye shall be remembered before the LORD your God, and ye shall be saved from your enemies.
God appointed the trumpets to give Israel clear signals that directed the movements of the people. The alarm of the verse belongs to going to war in their own land against the enemy oppressing them, and when Israel sounded it they would be remembered before the Lord their God and saved from their enemies.
The trumpets were part of God's system of order and discipline for Israel. The sons of Aaron carried the trumpets, received directions from Moses, and made them known to the people by speaking through the trumpets. The sounds they gave were special and certain, and the people knew and understood them to direct their movements. A special signal first called for attention, then all were to be attentive and obey, with no confusion of sound so that there was no excuse for confusion of movement. The head officer of each company gave definite directions, so that no one who gave attention was left ignorant of what to do.
Failure to comply with the requirements given to Moses and through Moses to the people was punished with death. Not knowing was no excuse, because all had the same opportunity to learn. The arrangement teaches that God is a God of order and is as particular now about order and organization as in the days of Moses.